This question comes out of the Parable of the wedding feast in Matthew 22:1-13, where we are told that the king found a man at the wedding feast who did not wear wedding clothes and whom the king cast out. We must evaluate the sin of this man against the background of the custom of his times. It was customary for one who invited guests to a feast, to supply a feast garment to every guest. This man apparently refused the feast garment and insulted the king by doing so, or he slipped into the feast as an uninvited gate crasher.
The wedding clothes are a symbolic presentation of our cleansing by the blood of the Lord Jesus. We can only go to the Father through Him, clothed with His righteousness and not ours (John 14:6: Jesus is: “The way, the truth, and the life”; no one goes to the Father except by Him). Whoever tries to enter heaven and the wedding feast of the Lamb (Revelation 19:5-9) by another way, will be cast out and be lost forever.
The wedding garment is therefore not a garment with which we clothe our bodies, like church clothes. It is a symbol of our salvation through Jesus only. Today we may receive these “wedding clothes” free of charge, but one day, when the King comes to inspect his guests, it will be too late to change.